1. Field of the Invention
The present invention relates generally to electrochemical fuel cell systems, and, more particularly, to fuel cell systems comprising a fuel cell stack and at least one hydration sensor apparatus for measuring membrane hydration in the fuel cell stack.
2. Description of the Related Art
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst, disposed at the interfaces between the electrolyte and the electrodes, typically induces the desired electrochemical reactions at the electrodes. The location of the electrocatalyst generally defines the electrochemically active area.
One type of electrochemical fuel cell is the polymer electrolyte membrane (PEM) fuel cell. PEM fuel cells generally employ a membrane electrode assembly (MEA) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two electrodes. Each electrode typically comprises a porous, electrically conductive substrate, such as carbon fiber paper or carbon cloth, which provides structural support to the membrane and serves as a fluid diffusion layer. The membrane is ion conductive (typically proton conductive), and acts both as a barrier for isolating the reactant streams from each other and as an electrical insulator between the two electrodes. A typical commercial PEM is a sulfonated perfluorocarbon membrane sold by E.I. Du Pont de Nemours and Company under the trade designation NAFION®. The electrocatalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support).
In a fuel cell, a MEA is typically interposed between two separator plates that are substantially impermeable to the reactant fluid streams. The plates typically act as current collectors and provide support for the MEA. In addition, the plates may have reactant channels formed therein and act as flow field plates providing access for the reactant fluid streams to the respective porous electrodes and providing for the removal of reaction products formed during operation of the fuel cell.
In a fuel cell stack, a plurality of fuel cells are connected together, typically in series, to increase the overall output power of the assembly. In such an arrangement, one side of a given separator plate may serve as an anode flow field plate for one cell and the other side of the plate may serve as the cathode flow field plate for the adjacent cell. In this arrangement, the plates may be referred to as bipolar plates. Typically, a plurality of inlet ports, supply manifolds, exhaust manifolds and outlet ports are utilized to direct the reactant fluid to the reactant channels in the flow field plates.
A broad range of reactants can be used in PEM fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
During normal operation of a PEM fuel cell, fuel is electrochemically oxidized on the anode side, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the membrane, to electrochemically react with the oxidant on the cathode side. The electrons travel through an external circuit providing useable power and then react with the protons and oxidant on the cathode side to generate water reaction product.
Water management issues are critical in PEM fuel cell operation. As the water content of the membrane falls, it loses the ability to transport protons, its electrical resistance increases, fuel cell performance decreases and membrane failure may occur. Accordingly, to ensure adequate humidification of the membrane, one or both of the reactant streams supplied to the fuel cell stack are typically humidified. However, if the water content of the membrane becomes too great, through, for example, excessive humidification or accumulation of water reaction product, the membrane may become flooded, thereby disturbing the diffusion of reactants and also resulting in a decrease in fuel cell performance.
Various different systems and methods have been developed for monitoring and controlling MEA humidification, or hydration, levels in fuel cell stacks. Typically, fuel cell resistance is calculated from voltage and current measurements and then, since the fuel cell resistance varies as a function of the humidity level of the fuel cell, the humidity level of the fuel cell is determined. Alternatively, a sensor may be used to measure the relative hydration of a reactant stream circulated to a fuel cell stack and, based upon this measurement, the hydration level of the MEAs and, implicitly, the membranes may be evaluated. For example, U.S. Patent Application No. 2003/0141188 discloses a hydration sensor comprising a fuel cell having a first electrode exposed to a measurement gas, the gas for which the moisture content is to be determined, and a second electrode exposed to a reference gas, a gas for which the moisture content is known. By monitoring the voltage and current of such fuel cell, the hydration level of the measurement gas may be determined. However, such a sensor further requires that a reference gas, separate from the measurement gas, also be provided to the sensor. As a result a fuel cell system comprising such a sensor becomes more complex.
Accordingly, although there have been advances in the field, there remains a need for improved and simplified systems and methods for monitoring and controlling membrane hydration levels in fuel cell stacks. The present invention addresses these needs and provides further related advantages.